Security and identification often relies on fingerprint data. Accordingly, sensors for providing fingerprint images have been under development for some time. Many such sensors employ an array of sensor pixels making direct contact with a finger. Readout of the sensor pixels provides fingerprint image data. Known approaches for providing such sensor pixels include capacitive sensing (e.g., as in U.S. Pat. No. 6,694,269), and temperature or pressure sensing (e.g., as in U.S. Pat. No. 4,429,413). Further references relating to pressure sensing include: U.S. Pat. No. 5,844,287, U.S. Pat. No. 6,672,174, U.S. Pat. No. 6,578,436, and US 2002/0121145.
In many cases, sensors based on pressure sensing include a flexible membrane that conforms to the valleys and ridges of an applied fingerprint. The membrane is typically suspended above a rigid substrate that provides mechanical support. Each sensor pixel is responsive to a separation between the membrane and substrate. The substrate often includes integrated electronic circuitry (e.g., pixel addressing circuitry). Known examples of this general approach include: U.S. Pat. No. 4,577,345, U.S. Pat. No. 5,745,046, U.S. Pat. No. 5,400,662, and U.S. Pat. No. 5,079,949. In practice, implementation of such sensor approaches is often excessively costly. A typical integrated fingerprint sensor chip dimension is 15 mm×15 mm to accommodate the size of a normal fingerprint and the area of the integrated processing circuitry. Such large chips are costly to fabricate, since the number of chips per semiconductor wafer is relatively low. Furthermore, sensors having rigid and breakable substrates (e.g., conventional silicon substrates) cannot be used for applications such as smart credit/identity cards where the sensor must survive a certain degree of flexure.
Another sensor approach is considered in an article by Young et al., entitled “Novel Fingerprint Scanning Arrays Using Polysilicon TFTs on Glass and Polymer Substrates”, and published in IEEE Electron Device Letters 18(1), pp 19-20, January 1997. In this work, the substrate is flexible, alleviating the above-mentioned breakage problem, and capacitive sensing is employed. Since capacitive sensing entails no significant relative motion of sensor parts in operation, mechanical complications resulting from substrate flexure are presumably avoided. However, the capacitive sensing in this work relies on integrated thin film transistors to amplify signals. Although thin film transistors deposited on flexible substrates are known (e.g., as in U.S. Pat. No. 6,680,485), it would be preferable to avoid the use of active devices integrated with the sensor array in order to reduce cost. In addition, the polymeric transistors used in such works can be unreliable in commonly encountered environmental conditions such as high humidity (which causes polymer transistor degradation). Furthermore, the fabrication of more traditional transistors, such as thin film transistors, requires exposure of the substrate to high temperatures during processing, which can cause degradation of typical polymer based flexible substrates.
Another flexible sensor is considered in an article by Engel et al., entitled “Development of polyimide flexible tactile sensor skin”, and published in the Journal of Micromechanics and Microengineering, 13, pp 359-366, 2003. In this work, each pixel includes a relatively thin membrane that flexes (or doesn't flex) responsive to the presence (or absence) of a fingerprint ridge. Flexure of the membrane is sensed by a strain gauge integrated with the membrane. Since the strain gauge is in the membrane, the substrate is not a functional part of each pixel. Instead, the substrate provides overall mechanical support, and may include circuitry for reading out the sensor array. A disadvantage of this approach is that the strain gauge output is analog. It is often preferred for fingerprint sensors to provide inherently digital outputs, since a digital image is often required in practice and post-conversion of an analog sensor image to a binary image is frequently error-prone.
Accordingly, it would be an advance in the art to provide a flexible fingerprint sensor overcoming the above-identified shortcomings. More specifically, a flexible fingerprint sensor providing an inherently binary output would be an advance in the art. A further advance in the art would be a flexible fingerprint sensor providing an inherently binary output and having only passive components in the sensor array.